Local Anesthesia & Pain Control

6.3 Local Anesthesia & Pain Control

Local Anesthetic Agents: Chemistry and Pharmacology

Local anesthetics block nerve conduction by preventing sodium influx through sodium channels in the nerve membrane. They are classified chemically into amides and esters.

Amides vs. Esters

• Amides: Contain an amide linkage. They are metabolized primarily in the liver (except articaine) and have an extremely low rate of allergic reactions.
• Esters: Contain an ester linkage. They are metabolized in blood plasma by the enzyme pseudocholinesterase. They have a higher rate of allergic reactions, primarily due to their metabolite, Para-Aminobenzoic Acid (PABA).

Amide Local Anesthetics in Clinical Practice

1. Lidocaine (Xylocaine): 2% concentration, with 1:100,000 or 1:50,000 epinephrine. It is the gold standard of local anesthetics, providing intermediate duration (~60 minutes pulpal, 3-5 hours soft tissue).
2. Articaine (Septocaine): 4% concentration, with 1:100,000 or 1:200,000 epinephrine. It is a hybrid molecule containing both an amide linkage and an ester group. Because of this, it is rapidly metabolized in the blood plasma (90%) by plasma cholinesterases, with only 10% metabolized in the liver. It has superior lipid solubility, allowing excellent diffusion through bone. It carries a slightly higher risk of paresthesia (particularly in mandibular blocks) due to the 4% concentration.
3. Mepivacaine (Carbocaine/Polocaine): 3% plain (no vasoconstrictor) or 2% with 1:200,000 levonordefrin. The 3% plain formulation is excellent for short procedures or for patients in whom vasoconstrictors are contraindicated.
4. Prilocaine (Citanest): 4% plain or 4% with 1:200,000 epinephrine. It is metabolized in the liver and lungs. Its metabolite, orthotoluidine, can cause methemoglobinemia, a condition that reduces the oxygen-carrying capacity of the blood. It is contraindicated in patients with congenital methemoglobinemia, sickle cell anemia, or those taking daily acetaminophen.
5. Bupivacaine (Marcaine): 0.5% concentration, with 1:200,000 epinephrine. It is a long-acting anesthetic (~90-180 minutes pulpal, up to 12 hours soft tissue) indicated for long procedures or post-operative pain management.

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Vasoconstrictors in Local Anesthesia

Vasoconstrictors (primarily epinephrine) are added to local anesthetics to constrict blood vessels, thereby decreasing systemic absorption (which reduces toxicity), prolonging the duration of anesthesia, and providing localized hemostasis.

Maximum Safe Dosages

Epinephrine dosages must be carefully calculated, especially for patients with compromised cardiovascular systems:

• Healthy Patient (ASA I or II): The maximum recommended dose of epinephrine is 0.2 mg per appointment.
  • This is equivalent to ~11 cartridges of 1:100,000 epinephrine (0.018 mg/cartridge) or ~22 cartridges of 1:200,000 epinephrine (0.009 mg/cartridge).
• Cardiac Patient (ASA III or IV, e.g., history of myocardial infarction within 6 months, unstable angina, uncontrolled arrhythmias, or uncontrolled hypertension): The maximum recommended dose of epinephrine is 0.04 mg per appointment.
  • This is equivalent to ~2 cartridges of 1:100,000 epinephrine or ~4 cartridges of 1:200,000 epinephrine.

Contraindications and Drug Interactions

• Absolute Contraindications: Recent myocardial infarction (within 6 months), unstable angina, uncontrolled hyperthyroidism (risk of thyroid storm), and uncontrolled arrhythmias.
• Drug Interactions:
  • Tricyclic Antidepressants (TCAs, e.g., amitriptyline): Can lead to a severe, life-threatening hypertensive crisis due to unopposed alpha-1 receptor stimulation.
  • Non-Selective Beta-Blockers (e.g., propranolol): Can cause significant hypertension and reflex bradycardia.

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Injection Techniques and Anatomical Landmarks

• Local Infiltration: Anesthetic is deposited near terminal nerve endings. Highly effective in the maxilla due to its porous bone plate. Ineffective in the mandibular posterior region due to thick cortical bone.
• Inferior Alveolar (IA) Block: Targets the inferior alveolar nerve as it enters the mandibular foramen. Anesthetizes mandibular teeth to the midline, the buccal periodontium of premolars/anteriors, lingual periodontium, and the anterior 2/3 of the tongue. It has a high rate of positive aspiration (10-15%).
• Posterior Superior Alveolar (PSA) Block: Targets the PSA nerve near the maxillary tuberosity. Anesthetizes the maxillary molars (excluding the mesiobuccal root of the first molar in 28% of patients, which requires an MSA block). It carries a high risk of hematoma if the needle punctures the pterygoid venous plexus.
• Middle Superior Alveolar (MSA) Block: Anesthetizes maxillary premolars and the mesiobuccal root of the first molar.
• Anterior Superior Alveolar (ASA) Block: Anesthetizes the maxillary canine, lateral, and central incisors and facial periodontium.
• Greater Palatine (GP) Block: Anesthetizes the posterior two-thirds of the hard palate and overlying soft tissue, from the first premolar distally.
• Nasopalatine (NP) Block: Anesthetizes the anterior one-third of the hard palate (canine to canine). This is a highly painful injection due to the tight attachment of the palatal mucosa to the bone.

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Systemic Toxicity and Local Complications

• Local Anesthetic Systemic Toxicity (LAST): Caused by elevated blood levels of anesthetic, usually due to accidental intravascular injection or exceeding the maximum recommended dose.
  • Symptoms: Initial excitation (talkativeness, anxiety, elevated BP/HR, tremors, shivering) followed by depression (confusion, seizures, loss of consciousness, respiratory depression, cardiovascular collapse).
  • Management: Stop treatment, position the patient supine, administer oxygen, monitor vitals, and activate EMS if seizures occur.
• Hematoma: Effusion of blood into the tissue space. Most common with PSA and IA blocks. Management involves applying immediate direct pressure for at least 2 minutes, placing ice packs for the first 24 hours (vasoconstriction), and applying warm compresses after 24 hours to promote resorption.
• Trismus: Spasm of the masticatory muscles, limiting jaw opening. Management: Warm compresses, warm saline rinses, analgesics (NSAIDs), and gentle jaw opening exercises.
• Paresthesia: Prolonged numbness or abnormal sensation. Caused by direct needle trauma to the nerve or contaminated anesthetic cartridges. Usually resolves spontaneously within 8 weeks.